The prior art is replete with sensors, systems, and medical devices that are designed to measure, process, monitor, and/or display physiological characteristics of a patient. For example, the prior art includes glucose sensor devices and systems that monitor blood glucose levels in a subject's body on a continuing basis. Presently, a patient can measure his/her blood glucose (BG) using a BG measurement device, which may be: a glucose meter such as a test strip meter; a continuous glucose measurement system or monitor; a hospital hemacue; or the like. BG measurement devices use various methods to measure the BG level of a patient, such as a sample of the patient's blood, a sensor in contact with a bodily fluid, an optical sensor, an enzymatic sensor, or a fluorescent sensor. When the BG measurement device has generated a BG measurement, the measurement can be output, displayed, processed, or otherwise handled in an appropriate manner.
Currently known continuous glucose measurement systems include subcutaneous (or short-term) sensors and implantable (or long-term) sensors. The current state of the art in continuous glucose monitoring (CGM) is largely adjunctive, meaning that the readings provided by a CGM device (including, e.g., an implantable or subcutaneous sensor) cannot be used without a reference value in order to make a clinical decision. The reference value, in turn, must be obtained from a blood sample, which may be obtained from a BG meter (such as a finger stick device). The reference value can be used to check the accuracy of the sensor, and it can also be used to generate a calibration factor that is applied to the raw sensor data.
The art has searched for ways to eliminate or, at the very least, minimize, the number of finger stick measurements that are necessary for calibration and for assessing sensor health. However, given the number and level of complexity of the multitude of sensor operating modes, no satisfactory solution has been found. At most, diagnostics have been developed that are based on either direct assessment of the sensor output current (Isig), or on comparison of two Isig values. In either case, because the Isig tracks the level of glucose in the body, by definition, it is not analyte independent. As such, by itself, the Isig is not a reliable source of information for sensor diagnostics, nor is it a reliable predictor for continued sensor performance.
Accordingly, it is desirable to have an improved physiological characteristic sensor and related sensor system that addresses the shortcomings of traditional sensor systems. In addition, it is desirable to have a calibration free BG sensor that need not rely on BG finger stick samples. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.